We propose to assess the oncogenic risk of low-dose-rate Gamma -irradiation relative to the risk of irradiation at high dose rate (36 Gy/hr). The risk will be measured by comparing the proportion of malignantly transformed C3H/10T1/2 fibroblasts following treatments of equal doses (0.25 Gy to 4 Gy) and at dose rates ranging from 0.05 Gy/hr to 8 Gy/hr. Dose rates will be proportional to total dose (constant exposure time). Exposure times will range from 0.5 to 5 hrs. Malignant transformation will be determined from clonal morphology and verified by murine tumor induction. The experiments will be performed with exponential cultures held at 37.5 C during protracted irradiation. High- and low-dose-rate irradiations will be performed with a suitably modified gamma irradiator.